Liquid crystal display (LCD) devices are widely used as a monitor for notebook computers and desktop computers and a television because of their high resolution, high contrast ratio, color rendering capability and superior performance for displaying moving images. An LCD device relies on the optical anisotropy and polarizing properties of liquid crystal to produce an image. A typical LCD device includes a liquid crystal display panel including two substrates and a liquid crystal layer between the two substrates. An electric field generated between the two substrates adjusts an alignment direction of liquid crystal molecules in the liquid crystal layer to produce differences in transmittance.
Because the liquid crystal display panel does not include an emissive element, a light source is required to view images on the liquid crystal display panel. Accordingly, a backlight unit having a light source is disposed under the liquid crystal display panel. The backlight unit for an LCD device may be classified as either a side light type or a direct type according to the position of the light source relative to the LCD panel. In a side light type backlight unit, light emitted from at least one side portion of the liquid crystal display panel is redirected by a light guide plate (LGP) to enter the liquid crystal display panel. In a direct type backlight unit, a plurality of light sources is disposed at a rear surface of the liquid crystal display panel so that light from the plurality of light sources directly enters the liquid crystal display panel.
A cold cathode fluorescent lamp (CCFL) and an external electrode fluorescent lamp (EEFL) have been used as a light source of a backlight unit for a liquid crystal display device. More recently, a light emitting diode (LED) has been used as a light source because the LED has a small size, low power consumption and a high reliability.
FIG. 1 is a cross-sectional view showing a liquid crystal display device including a light emitting diode as a light source according to the related art.
In FIG. 1, a liquid crystal display (LCD) device includes a liquid crystal panel 10, a backlight unit 20, a main frame 30, a top frame 40 and a bottom frame 50. The liquid crystal panel 10 includes first and second substrates 12 and 14 facing and spaced apart from each other and a liquid crystal layer (not shown) between the first and second substrates 12 and 14. Polarizing plates 19a and 19b are formed on outer surfaces of the liquid crystal panel 10.
The backlight unit 20 is disposed under the liquid crystal panel 10. The backlight unit 20 includes a light emitting diode (LED) assembly 29, a reflecting plate 25, a light guide plate 23 and a plurality of optical sheets 21. The LED assembly 29 is disposed along at least one side of the main frame 30, and the reflecting plate 25 having a white color or a silver color is disposed over the bottom frame 50. The light guide plate 23 and the plurality of optical sheets 21 are sequentially disposed over the reflecting plate 25. The LED assembly 29 includes a plurality of LEDs 29a and an LED printed circuit board (PCB) 29b. The plurality of LEDs 29a on the LED PCB 29b are disposed at one side of the light guide plate 23 and emit a white colored light.
The liquid crystal panel 10 and the backlight unit 20 are surrounded by the main frame 30 having a rectangular ring shape, and the top frame 40 surrounding a front boundary portion of the liquid crystal panel 10 and the bottom frame 50 covering a rear surface of the backlight unit 20 are combined with the main frame 30 so that the liquid crystal panel 10 and the backlight unit 20 can be coupled.
FIG. 2 is a magnified cross-sectional view of portion A of FIG. 1.
In FIG. 2, the plurality of LEDs 29a are disposed at one side of the light guide plate 23 and are formed on the LED PCB 29b to constitute an LED assembly 29. The LED assembly 29 is fixed by an attachment so that the light from the plurality of LEDs 29a can face a light-entering portion of the light guide plate 23. Accordingly, an edge of the bottom frame 50 is bent upwardly and then inwardly to have a bent portion 51, and the LED assembly 29 is fixed to the bent portion 51 with an adhesive material such as a dual coated tape. The LED assembly 29 may be referred to as a side top-view type.
As a result, the light F emitted from each of the plurality of LEDs 29a is inputted into the light guide plate 23 through the light-entering portion and is refracted in the light guide plate 23 toward the liquid crystal panel 10. The light refracted in the light guide plate 23 and the light reflected by the reflecting plate 25 are processed as a uniform plane light source of high quality while passing through the plurality of optical sheets 21 and are supplied to the liquid crystal panel 10.
In addition, the bent portion 51 of the bottom frame 50 and the light guide plate 23 are spaced apart from each other by a gap G, and a portion of the light F emitted from the plurality of LEDs 29a leaks through the gap G to cause a light leakage. The light leakage deteriorates the properties such as brightness and color uniformity of the LCD device. The bent portion 51 may be fabricated to minimize the gap G between the bent portion 51 of the bottom frame 50 and the light guide plate 23. However, as minimize the gap G between bent portion 51 and the light guide plate 23, the step of fixing the LED assembly 29 to the bent portion 51 becomes complicated.